Enterprises around the world require ways to protect against the interruption of their business activities which may occur due to events such as fires, natural disasters, or simply the failure of server computers or workstations that hold business-critical data. As data and information may be a company's most important asset, it is vital that systems are in place that enable a business to carry on its activities such that the loss of income during system downtime is minimised, and to prevent dissatisfied customers from taking their business elsewhere.
To achieve business continuity, it is necessary for such a system to be tolerant of software and hardware problems and faults. This is normally achieved by having redundant computers and mass storage devices such that a backup computer or disk drive is immediately available to take over in the event of a fault. Such a technique is described in Ohran et al., International patent application WO 95/03580. This document describes a fault-tolerant computer system that provides rapid recovery from a network file server failure through the use of a backup mass-storage devices. There are, however, a number of reasons why the techniques used by Ohran and others may be undesirable.
As can be seen from the Ohran, each server requiring a fault-tolerant mode of operation must be backed up by a near-duplicate hardware and software architecture. Such one-for-one duplication may make it infeasible and uneconomic to run a redundant network file server instead of a normal network file server. Further, the need for a redundant network to be continuously on-line to ensure that it is updated at the same time as the normal network server renders its use as an off-line facility for testing infeasible. In addition to this, the majority of redundant networks are unable to provide protection against application failure and data corruption because they either use a common data source (e.g. the same disks) or they use live data. Also, the majority of redundant networks are unable to provide for test and maintenance access without the risk of either network downtime or loss of resilience of the network.
The present invention aims to overcome at least some of the problems described above.